1. Technical Field
This disclosure relates to undesirable airway and drape fires which may erupt during surgical procedures.
2. Description of Related Art
Dangerous fires can and do occur during surgery, including in patient airways and under surgical drapes.
An airway fire is a fire in the airway or breathing circuit of a patient. Airway fires may be low-frequency, peri-anesthetic adverse events that may occur in healthy patients with devastating consequences, including severe burns, disfigurement, and death. Studies have established that incendiary characteristics and breakdown products of polyvinyl chloride (PVC) endotracheal tubes (ETTs), as well as the clinical scenarios, can lead to an airway or surgical field fire.
In a closed-claims review by an American Society of Anesthesiologists (ASA) Committee, the number of operating room (O.R.) fires was estimated to be between 50 to 200 operating room fires per year, with as many as 20% of the reported fires resulting in serious injury or death. However, the ECRI Institute, a nonprofit health services research organization, in collaboration with the Anesthesia Patient Safety Foundation, has reported that the estimate is more correctly between 500-650 cases of operating room fires per year. Because of medico-legal reasons, many fires may not be reported in the open literature. Thus, the true incidence may likely be higher.
As an example, one operating room fire occurred during an attempted Burr Hole procedure in a patient receiving 6 L O2 per minute through a face mask. Upon activation of a monopolar electrocautery surgical unit (ESU), a muffled ‘pop’ was heard, followed by the appearance of smoke under the drapes. When the drape was removed, the patient's head was reportedly engulfed in a “ball of flame.” The oxygen mask was also observed to be in flames. The fire was extinguished within 15 seconds. Still, the patient sustained second degree burns to the face, neck, and upper chest, complicated by pneumonia and a two-month hospitalization. The sequence of events leading to the fire was simulated with a manikin. The causes of the fire were considered to be three-fold: ESU ignition, an enriched oxygen environment provided by the face mask in a tented closed space, and fuel provided by vapors from an alcohol-based preparation solution.
Fire Sources in the Operating Room
Three elements must usually be present for a fire: an ignition source, an oxidizer, and a fuel source. The categorical causes may be broadly described as:                Ignition sources in the O.R. setting include electrocautery, electrosurgical units, lasers, heated probes, drills or burrs (heat or sparks), argon beam collimators, fiber-optic light cables, defibrillation pads or paddles, and other heat-generating or flame-generating devices.        Gaseous oxidizers include oxygen and nitrous oxide, the latter being a potent oxidizer. Oxidizer enriched environments can be created internally within a closed or semi-closed (circle system) breathing circuit, the endotracheal tube (ETT)/laryngeal mask airway (LMA), the lower airway below the vocal cords, and in any breathing tube that serves as a conduit for delivery of oxygen to the lungs. Other oxygen delivery systems in use include tracheostomy tubes, double-lumen tubes for separate and/or combined lung ventilation, transtracheal oxygen jet devices, and endoscopes equipped with channels for gas delivery. An oxidizer-rich environment can also be created externally with the use of open gas sources (e.g., nasal cannulae, external face masks, tracheostomy masks), particularly when combined with drapes and tenting environments that promote the pooling of oxygen or nitrous oxide.        Potential fuel sources include (1) Patient: hair, gastrointestinal gases; (2) Surgical preparation agents: alcohol, degreasers (acetone), aerosols, tinctures (benzoin, mastazol), ethyl chloride spray, dermatone glue; (3) Linens: gowns, drapes, blankets, paper materials; (4) Dressings: stockinette, tapes, sponges, collodion, gauze; (5) Ointments: wax, medical adhesive spray, petrolatum, tincture of benzoin, plastic and rubber products; (6) Anesthesia components: Breathing/respiratory circuits, mask, airways, ETTs, carbon dioxide absorbents. Flammable agents (ether) are no longer used in the USA and other modern countries.        
With use of an ETT, there are two possible regions within which an airway fire can originate—either within the ETT lumen, or external to the ETT. Airway fires can occur even without an ETT, such as with nasal cannulas and face masks.
For surgery cases requiring a sterile cover sheet, an ignitable environment can be created in a closed tented space underneath the sheet. This scenario can arise in multiple ways, such as when (a) an oxygen-enriched space is created underneath the sheet with supplementary oxygen given to a non-intubated patient, or (b) when there is an accumulation of ignitable vapors from incompletely dried surgical prep solutions. Many other potential fire scenarios exist.
Fire Prevention and the ASA Response Algorithm
The International Organization for Standardization, Anesthesia Patient Safety Foundation, and the American Society of Anesthesiologists (ASA) have developed educational programs to minimize the incidence of OR fires, and to improve the quality and speed of the OR team response. If an airway fire erupts, for example, the ASA algorithm requires the following: the ETT is immediately removed, the flow of airway gases is stopped, flammable materials are removed, and a bowl of saline is poured into the airway.
However, a sudden outburst of fire may startle most individuals. Despite prior drills, the instinctive response of a team member may be to back away initially from the danger. Seconds can matter in an airway burn, and it may take seconds for the anesthesiologist to turn off the oxygen, remove the tape, and pull out the ETT which may be on fire. The saline may not have been poured into a vessel, or could be spilled, thus making it unavailable for use by the surgeon.
Flame Spread in Tracheal Tubes
Upon ignition of a tracheal tube, there are three types of flames that can occur: (1) extraluminal outer surface flames that can arise when the O2 concentration outside the tube exceeds the O2 flammability index, (2) intraluminal upstream flames and (3) intraluminal downstream flames.
Model studies have demonstrated that, when the free distal end of an ETT is ignited by a pilot flame, if the O2 concentration is sufficiently high, an intraluminal upstream flame develops and spreads against the direction of the oxygen flow toward the supply of oxidizer at the proximal end of the tube. The flame may spread at a speed of 1.5 to 2 cm/sec at O2 flow rates of 2-5 liters/min.
A third type of flame, the intraluminal downstream flame, feeds off the excess un-reacted gaseous fuel generated by the upstream flame. It is this downstream flame anchored distally that is regarded as the most dangerous.
In general, extraluminal flames tend to be mild, while intraluminal (and particularly downstream) flames can be violent owing to the forced supply of oxidizer with high O2 concentration and high total flow rates. When a laser beam strikes the outside wall of an ETT, an extraluminal fire can be created, and with penetration through the wall, an intraluminal flame as well.
Vulnerability of Laser Shields
Several manufacturers have designed tracheal tubes to be resistant to the effects of a laser, such as the Laser-Shield II tracheal tube, Norton tube, Lasertubus, and Sheridan Laser Trach Tube. However, each may have limitations. In an earlier version of the silicone-based Xomed Laser Shield ETT (externally coated with a layer of metal particles), for example, the shaft of the Xomed ETT in a 100% environment could be ignited with long-term exposure to a laser beam, with conversion into an intense “blowtorch.” The Laser-Flex ETT has a stainless steel shaft, but has a large outer diameter (difficult to use in small patients and children), small inner diameter (may limit ventilatory flow), and a vulnerable cuff. If the tube wall is heated above 160 degrees C., ‘hot spots’ may develop and the inner PVC cuff conduits may begin to disintegrate and can be ignited with Nd—YAG lasers.
The Non-Intubated Airway—A Standardized Protective Tent
For monitored anesthesia care not involving an ETT, supplementary O2 may be provided by a face mask or nasal cannula. With drapes over the patient's head, a tenting effect is created, and excess O2 gas may accumulate, particularly in drape folds, thus producing an ignitable environment. A high-energy laser beam can ignite the drape, and can ignite the underlying O2-enriched airway mask/cannula/anesthesia tubing, the patient's hair or clothing, or the bed linen.
The Emergency Care Research Institute (ECRI) reports that 60% of operating room fires involve a surgical drape as fuel and 40% occur in an oxygen-enriched environment. New drape materials made of polypropylene or phenol polymer may not ignite in air for 30 seconds in the presence of a 15 W carbon dioxide laser beam. However, a small hole may be created through the drape, which may secondarily ignite flammable material placed underneath it.
Further, there is no surgical drape material, either currently available or proposed, that is not ignitable in a 50% or 95% O2 environment.